The area of laser processing of materials encompasses a wide variety of applications that involve cutting, drilling, milling, welding, melting, etc. and different types of materials. Among these applications, one that is of particular interest is cutting or separating parts from different types of substrate materials, such as separating arbitrary shapes out of sapphire. Sapphire possesses exceptional hardness and toughness, properties that make it very resistant to scratching, and it is also highly transparent to wavelengths of light between 150 nm and 5500 nm.
Sapphire is used in several applications that rely on its unique and exceptional combination of electrical, mechanical, thermal, and optical properties. These applications include infrared optical components, such as in scientific instruments, high-durability windows, barcode scanners, wristwatch crystals and movement bearings, and very thin electronic wafers, which are used as the insulating substrates of special-purpose solid-state electronics (most of which are integrated circuits). Sapphire is also used in the semiconductor industry as a non-conducting substrate for the growth of devices based on gallium nitride (GaN). In particular, sapphire has low electrical conductivity, but a relatively high thermal conductivity. Thus, sapphire provides good electrical insulation, while at the same time helping to conduct away the significant heat that is generated in all operating integrated circuits. More recently, it has been offered as an alternative material for smartphone camera windows, screen cover and touch applications in consumer electronics products.
Because sapphire is very hard, one of the major challenges in manufacturing parts out of this substrate material is the cutting process. Typically, cutting can be achieved by first using a diamond-tipped blade to scribe a pattern in the substrate. After that, the scribed profile is either subjected to a mechanical force that propagates the crack into the substrate and along the traced profile to separate the part completely, or scribing is followed by a second pass of the circular diamond blade to cut through the substrate. The diamond blade has a small but finite width and the separation process reserves a “street” (typically greater than about 40 μm) between two parts to be separated to account for the width of the diamond blade. To maintain the quality of the edge of the part separated from the substrate and also to avoid catastrophic and uncontrolled cracking of the substrate, the diamond tip blade must be operated at a low speed, which prolongs the separation process. Also, because of the abrasion, the diamond tips on the blade wear out and must be replaced often—as much as one blade per wafer, which slows down the manufacturing process and increases costs. Finally, the mechanical scribing process causes cracks, which can damage the substrate and reduce yields (typical yields are claimed to be about 70%).
Another challenge regarding sapphire cutting and processing is related to shapes of separated parts. Due to the crystalline nature of sapphire, cleavage and separation preferentially occur in straight lines aligned with one of the crystal planes. However, this same feature makes it difficult to cut and separate sapphire parts having more complex shapes. For example, when separating a circular shape out of a square substrate, depending on the induced stress and the crystal alignment to the circular shape, crack propagation can deviate from the intended circular path and instead occur along a path of least resistance following one of the structural crystal planes.
From process development and cost perspectives, there are many opportunities for improvement in cutting and separation of sapphire substrates. It is of great interest to have a faster, cleaner, cheaper, more repeatable and more reliable method of sapphire separation than what is currently practiced in the market today. Among several alternative technologies, laser separation has been tried and demonstrated using different approaches. The techniques range from: 1) actual removal of material between the boundaries of the desired part (or parts) and its surrounding substrate matrix; 2) creation of defects within the bulk of the material to weaken or seed it with cracking initiation points along the perimeter of the desired shape profile followed by a secondary breaking step; and 3) propagation of an initial crack by thermal stress separation. These laser cutting processes have demonstrated the potential economic and technical advantages, such as precision, good edge finish, and low residual stress compared to competing technologies (mechanical scribing and breaking, high pressure water jet and ultrasonic milling, etc).
There is nevertheless a continuing need for an improved process for cutting and separating arbitrary shapes of sapphire.